Medication errors during transition of care are common; about 50% of the patients have medication discrepancy after discharge from hospitals, and medication error associated adverse drug events are a significant source of prevent harms. The long-term goal of our effort is to develop risk-informed tools for inpatient, transitional and outpatient care providers to effectively engage and support patients to improve patient safety. The overall objective of this project is to develop a patient-centric risk model of medication errors in ambulatory care. The variability and unsatisfactory results in outcomes of transitional care programs is an indication that our knowledge of medication safety in ambulatory care settings is insufficient and excludes potentially effective intervention strategies This project is based on the assumption that human factors approaches can contribute to the understanding of underlying hazards and mitigating strategies in medication safety from a patient-centered perspective, by conceptualizing the roles of health professionals as enhancing the patient work system and supporting the patient to achieve medication safety. The project's aims are: (1) Identify hazards and mitigating strategies to medication safety using a patient work system framework; (2) Develop a patient-centric risk assessment tool through prospective qualitative studies, and (3) Evaluate the risk assessment tool in a multi-site prospective study of hazards and mitigating factors medication safety and their associations with medication discrepancies and potential adverse events. Application of human factors approach is innovative to understand the mechanisms through which system factors increase and decrease the patient's ability to self-manage safely. The epidemiology of hazards and risks in the patient work system and their association with medication safety will enhance future development and deployment of improvement strategies, such as information technology tools, patient engagement and care system design, especially in the context of accountable care for the entire continuum of care. Our project will target the expected high medication error risk periods: care transitions of vulnerable patients with high risk medications. The research team is interdisciplinary with nationally known expertise in human factors, ambulatory care medication safety, patient safety, biostatistics, hospitalist program, and primary care patient safety. Multipe study sites will be used with an expected high representation of two AHRQ priority populations: the elderly and those with low socio-economic status. We also anticipate the value of the risk model to identify specific risk factors contributing to the disparities associated in medication safety.